Refactor producing cache keys in GHA jobs

[webknjaz]

This change makes the GHA cache dirs and keys calculated dynamically per recommendation @ #1378 (comment).

Changelog-friendly one-liner: $sbj (not user-facing)

Contributor checklist

• Provided the tests for the changes.
• Gave a clear one-line description in the PR (that the maintainers can add to CHANGELOG.md on release).
• Assign the PR to an existing or new milestone for the target version (following Semantic Versioning).

[atugushev]

I wonder can we put this script into a python file and run it instead?

[webknjaz]

I don't think it's a good idea. Moving the context out of the workflow makes it harder to follow and forces people to look up external sources. This would be different if it was needed outside of GHA, of course.

[webknjaz]

Also, hashFiles() is only accessible within workflows.

[webknjaz]

It would be possible to make it a composite action but it looks like an overkill.

[ssbarnea]

@webknjaz Please make it draft until is ready for review and merge.

[hugovk]

pip can now be cached via actions/setup-python to save a lot of config:

https://github.com/actions/setup-python#caching-packages-dependencies

Please see #1571 for an alternative.